Ultrasound has been used as a therapeutic technique in physical medicine for over forty years. By 1955, the Council on Physical Medicine and Rehabilitation of the American Medical Association recommended the technique as an adjunctive therapy for the treatment of pain, soft tissue injury, and joint dysfunction including osteoarthritis, periarthritis, bursitis, tenosynovitis, and a variety of musculoskeletal syndromes. Other applications such as acceleration of wound healing, phonophoresis of topical drugs, treatment of scar tissue, and treatment of sports injuries have been reported.
Ultrasonic therapy relies on mechanical vibration of tissue to cause thermal and nonthermal effects, using the conventional therapeutic frequency of 1 Mhz (megahertz) or the newer 3 Mhz frequency. Basically, the electrical output from the ultrasonic generator is converted into mechanical vibration through a transducer made generally of a crystalline material, such as lead zirconate titanate (PZT) or other synthetic or natural crystals. The mechanical vibration produces an acoustic wave which travels through the tissue and is absorbed in the process. The rate of absorption and, thus, the thermal effect is based on the tissue type encountered, the frequency of the ultrasound beam, and the intensity of the ultrasonic output. The energy is transferred from the transducer to the patient's tissue using a coupling medium or couplant, such as ultrasonic gel, lotion, hydrogel, or water. Output may be continuous wave or pulsed depending on the therapeutic indication. Output intensities of 0.1-3.0 W/cm.sup.2 (watts per square centimeter) are typically applied for therapeutic purposes in pulsed or continuous modes with ultrasound therapy.
Power output of an ultrasonic transducer may be indicated in watts as the total power emanating from the transducer or as intensity in W/cm.sup.2 which is the total acoustic power output divided by the effective radiating area or surface (ERA) of the transducer. Intensity is commonly used for therapeutic ultrasound applications. Continuous wave ultrasound output may also be measured in energy units of joules (watt-seconds) which is the amount of total delivered acoustic power applied to the tissue over the total treatment time. Continuous wave ultrasound is generally used for thermal applications.
Pulsed output is also referred to as amplitude modulated waveform operation. This means that the output turns on and off in short periodic intervals. This reduces the average power delivered to the patient, as compared to continuous output. Output power is displayed as the average power during the pulse of ultrasonic energy. In order to calculate the average power over time, the indicated power output is multiplied by a duty factor, which is defined as the on-time per period divided by the total time of the period, that is, duty factor=(time.sub.on)/(time.sub.on +time.sub.off). Pulsed outputs are generally used in pain control and tissue healing applications at non-thermal levels. Pulse durations and duty factors should be selected based on clinically tested parameters. Ultrasonic therapy systems should provide a full range of pulsed modes, allowing the clinician to treat acute as well as chronic injury.
In order to accurately portray the output characteristics of ultrasonic therapy transducers, an evaluation of the radiating surface must be made. This can be done using sophisticated testing systems which scan each portion of the transducer with an underwater microphone known as an acoustic hydrophone. Evaluation of the radiating surface of most transducers demonstrates that in the near field (generally within 10-30 cm of the sound head surface), significant irregularities in output intensity exist. These range from little or no intensity to very high peak intensities.
The effective radiating area (ERA) of an ultrasound transducer is determined by scanning the transducer at a distance of 5 mm (millimeters) from the radiating surface and recording all areas in excess of 5 per cent of the maximum power output found at any location on the surface of the transducer. The ERA is always smaller than the actual transducer surface; thus, the apparent size of the transducer is not indicative of the effective radiating surface. For this reason, individual scans of each transducer should be performed to ensure proper calibration of intensity in W/cm.sup.2 of ultrasonic output. A common mistake is to assume that the entire surface of an ultrasound transducer radiates ultrasound output. This is generally not true, particular with larger transducers, such as 10 cm.sup.2. There is no point in having a large transducer with a small effective radiating surface because such an arrangement mechanically limits the coupling in smaller areas. The transducer ERA should match the physical transducer head size as closely as possible for ease of application to various body surfaces in order to maintain the most effective coupling.
The major measure of beam homogeneity of an ultrasonic transducer is the beam nonuniformity ratio (BNR). Beams from practical ultrasound transducers are not homogeneous in intensity across their wavefronts. The BNR is the ratio of the highest intensity found in the field to the average intensity, as indicated on the output display of the generator. If, for example, the BNR of a transducer is 6:1, as is typical for many ultrasonic therapy transducers, and the intensity is set at 1.5 W/cm.sup.2, intensities in the order of 9.0 W/cm.sup.2 would be present in the field. The high peak intensities with high BNR's are responsible for the discomfort or periosteal pain often associated with ultrasound treatment.
The higher the BNR, the more important it is to move the transducer faster during treatment to avoid hot spots and areas of tissue damage or cavitation, which refers to microbubble implosions causing small vessel and cell membrane damage. Application technique is, thus, BNR dependent. The allowable intensity of the ultrasound beam should be the lower of patient tolerance to the beam intensity or the transient cavitation threshold of 8 W/cm.sup.2, which is calculated by multiplying the BNR by the output intensity. The lower the BNR, that is, the more even the spacial intensity of the beam, the slower the operator may apply the ultrasound to the treatment area without fear of periosteal burning or transient cavitation). Low BNR's provide highly uniform ultrasonic therapy fields, allowing rapid bulk heating, since the clinician is able to move slowly in the treatment area with a homogeneous beam. Accurate measurement of BNR also allows the clinician to select transducers which ensure consistent, uniform beam characteristics.
Absorption characteristics of ultrasound are unique in that the absorption coefficients for many tissues vary linearly with frequency over the range of 1.0 to 5.0 MHz. For this reason, transducers operating at both 1 and 3 MHz ultrasonic frequencies should be available to provide the clinician control the depth of beam and biological effects. Ultrasound energy at 1 MHz has a half value layer (50 per cent energy absorption layer) of 3.0 cm in muscle, whereas 3 MHz has a half value layer of 1.0 cm in muscle. The 3 MHz frequency is selected where the effect is required superficially. This would include reduction of edema in recent injuries and treatment of skin abrasions, bruising, epicondylitis, arthritis of small joints, scar tissue, periostitis, ulcers, and pressure sores. Acute injury responds well to 3 MHz ultrasound treatment, and there is a good analgesic effect. The lower frequency of 1 MHz common to most ultrasonic therapy generators is used to treat more deeply seated lesions where fibrotic or sclerotic conditions exist, widespread contractures in tendon and muscle, and large joints affected by osteoarthritis.
The biological effects of ultrasound may be categorized according to two major areas: thermal effects and nonthermal effects, including acoustic streaming, cavitation, and other mechanical effects.
In local regions of tissue subjected to an ultrasonic plane travelling wave, heat is produced at a rate per unit volume proportional to the level of intensity and the absorption coefficient of the tissue. Under stable conditions, temperature increases at a constant rate, provided the treatment area is limited to an area of approximately twice the ERA of the transducer. Treating too large of an area at a time will result in cooling effects by the normal blood flow, and an inadequate tissue temperature increase will result. Surprisingly, many therapeutic ultrasound treatments given every day in the clinic do not result in temperature increases of noteworthy value, although the clinician's intent was to provide thermal effects. This is due in part to the uneven distribution of ultrasound across the transducer surface (high BNR), the treatment of too large of an area for too short of a time, and the lack of consideration of tissue target location and coupling media.
Tissues with high collagen content most strongly absorb ultrasonic energy at the frequencies commonly used therapeutically. Most soft tissues have similar absorption coefficients. As previously described, beam penetration is a function of frequency. Thus, if one wishes deep heating, the use of 1 MHz ultrasound is indicated, as compared to surface where 3 MHz would be used.
The capability of obtaining reliable temperature increases in the tissue is the key to achieving therapeutic effects. Alterations in cell membrane diffusion, extensibility of collagen, and catalytic reactions times are specifically temperature dependent. In general, the application of therapeutic ultrasound is used to produce heating effects ranging from mild (1.degree. C. tissue temperature increase), moderate (2.degree. C.), or vigorous (4.degree. C.). The higher temperature increases are useful in the treatment of chronic connective tissue and joint diseases such as contracture, chronic scars, and osteoarthritis, whereas the lower temperature increases are used in the treatment of subacute injury and to accelerate tissue repair. It has been noted that thermal levels of ultrasound induce local release of histamine, further increasing blood flow and local microcirculation. Other researchers have demonstrated that the selective heating of nerve tissue may produce temporary conduction blocks in nerve action potential propagation. This may be a factor in blocking pain with ultrasonic therapy applied at thermal levels.
Regarding nonthermal effects, acoustic streaming is an effect produced by the ultrasonic beam within the tissue which occurs primarily at the cell membrane interface. Streaming is the unidirectional movement of tissue components exposed to the ultrasonic field. This effect has been observed by many investigators, and is thought to be responsible for affecting cellular diffusion rates, membrane permeability, and accelerated synthesis of collagen. It should be noted that these stimulatory effects generally occur at low intensity, pulsed modes of 3 MHz ultrasound and are not evident at higher intensities and different frequencies. The primary effects on collagen synthesis and healing rates appear to occur at an early stage of the regeneration process during active growth and proliferation. Reactions limited by diffusion rates including the re-absorption of exudates may be accelerated by streaming effects. These movements also contain enough energy to enhance the making or breaking of weak secondary hydrogen bonds, thus accelerating enzymatic reactions.
It has also been found that platelets exposed to ultrasonic fields release serotonin. This may help to explain pain reduction effects, since serotonin is an important neurotransmitter involved in the release of endogenous opiates, such as enkephalins and dynorphins. Increased phagocytic activity and bactericidal capacity have also been observed with five minute ultrasonic energy exposures at therapeutic levels of insonation.
Cavitation is a condition where gas bubbles form within the tissues as a result of ultrasonic radiation. These bubbles oscillate within the ultrasound field at the ultrasonic frequency. Stable bubble formation in vivo has been observed using acoustic imaging techniques at intensities in the therapeutic range at intermuscular interfaces. Low level cavitation and mechanical micromassage at the cellular level is thought to be responsible for cellular stimulation, edema reduction in post traumatic injury, as well as the stimulation of mechanoreceptors and the autonomic nervous system. Sympathetic, parasympathetic, and sensory stimulation by ultrasound in pulsed modes may account for the pain relieving effects often associated with ultrasonic therapy. Some researchers have demonstrated increased nerve conduction velocity following insonation of nerve tissues. It is apparent from many clinical observations that ultrasound produces significant analgesic effects in a large percentage of patients on account of both neurologic stimulation and the anti-inflammatory effects of ultrasound on tissues.
As described above, ultrasound therapy has measurable and otherwise verifiable thermal and nonthermal effects on various types of tissues depending on various parameters of the applied acoustic energy and manner of application to tissues. Thus, control of the parameters of therapeutic ultrasound would provide a clinician with the capability of treating a great variety of ailments with predictable results. However, the great majority of ultrasound treatments are applied with fixed parameters, usually 1 MHz at 1.5 W/cm.sup.2 for five minutes with a relatively high BNR transducer, regardless of the area or depth of intended tissue treatment. Although some nonthermal benefits may be derived from such an approach, the thermal benefits usually intended are seldom realized from such an imprecise application of ultrasound.
While clinicians, such as osteopaths, chiropractors, and physical therapists, and others who employ therapeutic ultrasound are usually knowledgeable in their respective areas of specialty, they are usually not trained to make the necessary adjustments to therapeutic ultrasound instruments which would enable the wide variety of treatments of which the application of ultrasound is capable. Therefore, an approach which would facilitate the control of ultrasound transducer parameters in relation to the type and location of tissue to be treated and the desired effect on the treated tissue would be of considerable benefit to the patient receiving the treatment.